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Abstract
Site-2-proteases (S2Ps) mediated proteolysis of transmembrane transcriptional regulators is a conserved mechanism to regulate transmembrane signaling. The universal presence of S2P homologs in different cyanobacterial genomes suggest conserved and fundamental functions, though limited data has been available. Here we provide the first evidence that Sll0528, a site-2-protease in Synechocystis sp. PCC 6803 is crucial for salt, cold and hyperosmotic stress acclimation. Remarkable induction of sll0528 gene expression was observed under salt, cold and hyperosmotic stress, much higher than induction of the other three S2Ps. Knock-out of sll0528 gene in wild type Synechocystis sp. PCC 6803 increased their sensitivity to salt, cold and hyperosmotic stress, as revealed by retarded growth, reduced pigments and disrupted photosystems. The sll0528 gene was induced to a much smaller extent by high light and mixotrophic growth with glucose. Similar growth responses of the sll0528 knockout mutant and wild type under high light and mixotrophic growth indicated that sll0528 was dispensable for these conditions. Recombinant Sll0528 protein could cleave beta-casein into smaller fragments. These results together suggest that the Sll0528 metalloprotease plays a role in the stress response and lays the foundation for further investigation of its mechanism, as well as providing hints for the functional analysis of other S2Ps in cyanobacteria.
Highlights
As the oldest and most abundant photosynthetic organisms, cyanobacteria play essential roles in ecology and biogeochemistry
The salt induced decrease of the photosystem I (PSI)/PSII ratio was much bigger in mutant than in wild type (Figure 3d), suggesting that the photosystem in the sll0528 mutant was seriously impaired upon salt stress
Hyperosmotic stress generated by 0.5 M sorbitol impeded the growth of wild type, while the sll0528 mutant was more sensitive to 0.5 M sorbitol and grew even more slowly (Figure 4a)
Summary
As the oldest and most abundant photosynthetic organisms, cyanobacteria (blue-green algae) play essential roles in ecology and biogeochemistry. Their stress responses are good examples for indicating how organisms adapt to an ever-changing environment. Among the known mechanisms of transmembrane signaling, the Site-2-proteases (S2Ps) mediated proteolysis of transmembrane transcriptional regulators is widespread and conserved among different organisms [5]. Human S2P activates cholesterol and fatty acid biosynthesis by cleaving transcription factor SREBP (Sterol Regulatory Element Binding Protein) after site-1 protease (S1P) cleavage [6,7]. Results indicated that the metalloprotease Sll0528 plays an essential role in salt, cold and hyperosmotic stress acclimation of Synechocystis
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